Chromosome Structure And Function 76g2s

 Chromosomes were first described by

Strausberger in 1875 as thread-like structure which appeared during cell division.

 The term “Chromosome”, however was first used

by Waldeyer in 1888.

 They were given the name chromosome (Chroma =

colour; Soma = body) due to their marked affinity for basic dyes.

 Chromosomes are composed of thin chromatin

threads called Chromatin fibers.

 These fibers undergo folding, coiling and

supercoiling during prophase so that the chromosomes become progressively thicker and smaller.

 Therefore, chromosomes become readily

observable under light microscope.

 At the end of cell division, on the other hand, the

fibers uncoil and extend as fine chromatin threads, which are not visible at light microscope

Chromosomes Can Be Circular or Linear Prokaryotic Chromosomes  Most of Prokaryotic cells have a single, circular chromosome.  numerous examples of prokaryotic cells that have

multiple chromosomes, linear chromosomes, or even both.  Borrelia are a notable exception to this arrangement, with bacteria such as Borrelia burgdorferi, the cause of Lyme disease, containing a single linear chromosome.

Eukaryotic Chromosomes  In contrast, all eukaryotic cells have multiple linear

chromosomes.  Circular DNA molecules also occur in mitochondria, which are present in almost all eukaryotic cells, and in chloroplasts, which are present in plants and some unicellular eukaryotes.  In eukaryotes, nuclear chromosomes are packaged by proteins into a condensed structure called chromatin.

Eukaryotic Chromosomes  The major components of chromatin are DNA and

histone proteins.

 two types of chromatin can be distinguished: • •

Euchromatin, which consists of DNA that is active, e.g., being expressed as protein. Heterochromatin, which consists of mostly inactive DNA.

Morphology of Prokaryotic Chromosomes  The genomes of prokaryotes

are contained in single chromosomes, which are usually circular DNA molecules.  Complexed with histone-like proteins in a structure termed the nucleoid.  "naked" DNA  Attached to plasma membrane

Plasmids  Prokaryotes also frequently carry one or more smaller independent 

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circular DNAs, called plasmids. Bacterial cells may also conatin plasmids that are autonomously self replicating extrachromosomal DNA that confer special characteristics to the cell in which it is present. Unlike the larger chromosomal DNA, plasmids typically are not essential for bacterial growth. Instead, they carry genes that confer desirable traits to the bacteria, such as antibiotic resistance. Also distinct from chromosomal DNA, plasmids can be present in many complete copies per cell. Eg. Antiobiotic Resistance genes  Plasmids include the fertility factor (F+ plasmid)

open circular chromosomes

covalently closed circular chromosomes

 Thus, one way prokaryotes compress their DNA into

smaller spaces is through supercoiling.  Genomes can be negatively supercoiled, meaning that the

DNA is twisted in the opposite direction of the double helix, or positively supercoiled, meaning that the DNA is twisted in the same direction as the double helix.  Most bacterial genomes are negatively supercoiled during

normal growth.

Supercoiling

Most common type of supercoiling

Helix twists on itself in the opposite direction; twists to the left

The Bacterial Chromosome Is Condensed Into Chromosomal Domains  The circular DNA is packaged into a region of the cell

called the nucleoid where it is organized into 50 or so loops or domains that are bound to a central protein scaffold, attached to the cell membrane.

Bacterial Chromosome  Single, circular DNA molecule located in the nucleoid

region of cell

Bacterial Chromosome  DNA gyrase is necessary for the unwinding the

coils.  Topoisomerases are enzymes that unwind and

wind DNA, in order for DNA to control the synthesis of proteins, and in order for DNA to reproduce. They cut the DNA, and at the end of the process connect it again

Supercoiling

Eukaryotic Chromosome  Eukaryotes possess multiple large linear chromosomes

contained in the cell's nucleus.  DNA Is Organized into Chromatin in Eukaryotes.

 The complexes between eukaryotic DNA and proteins

(histone and non-histone proteins) are called Chromatin, which typically contains about twice as much protein as DNA.  Complex interactions between proteins and nucleic acids in

the chromosomes regulate gene and chromosomal function

Chromonema  A chromosome consists of

two chromatids and each chromatid consists of thread like coiled structures called chromonema (plural chromonemata).  The term chromonema was coined by Vejdovsky in 1912.  The chromonemata form the gene bearing portion of chromosomes.

Matrix  The mass of achromatic material which surrounds

the chromonemata is called matrix.  The matrix is enclosed in a sheath which is known as pellicle. Both matrix and pellicle are non genetic materials and appear only at metaphase, when the nucleolus disappears.

Human Chromosomal DNA Content During the Cell Cycle

N = the number of different chromosomes in a nucleated cell.. C = the DNA content For humans N = 23; C = ~3.5 pg Ploidy – refers to the number of copies of chromosomes Most human cells are diploid 2n and 2C (somatic cells) Sperm and egg cells are haploid (n and C) (gametes).

Chromatin  The DNA of eukaryotic cell is tightly bound to small basic

proteins (histones) that package the DNA in an orderly way in the cell nucleus.  For e.g., the total extended length of DNA in a human cell

is nearly 2 m, but this must be fit into a nucleus with a diameter of only 5 to 10µm.  The major proteins of chromatin are the histones – small

proteins containing a high proportion of basic amino acids (arginine and lysine) that facilitate binding negatively charged DNA molecule .  There are 5 major types of histones: H1, H2A, H2B, H3,

and H4 – which are very similar among different species of eukaryotes.

The major histone proteins: Histone Mol. Wt H1 H2A H2B H3 H4

22,500 13,960 13,774 15,273 11,236

No. of Amino acid 244 129 125 135 102

Percentage Lys + Arg 30.8 20.2 22.4 22.9 24.5

The DNA double helix is bound to proteins called histones. The histones have positively charged (basic) amino acids to bind the negatively charged (acidic) DNA. Here is an SDS gel of histone proteins, separated by size

 In addition, chromatin contains an approximately

equal mass of a wide variety of non-histone chromosomal proteins.  There are more than a thousand different types of these proteins, which are involved in a range of activities, including DNA replication and gene expression.  The DNA of prokaryotes is similarly associated with proteins, some of which presumably function as histones do, packing the DNA within the bacterial cell.  Histones, however are unique feature of eukaryotic cells and are responsible for distinct structural organization of eukaryotic chromatin only.

Morphology of chromosomes. Satellite

Arm Centromere (primary constriction)

Secondary constriction

Telomere

Chromatids

Centromeres and Telomeres  Centromeres and telomeres are two essential features

of all eukaryotic chromosomes.  Each provide a unique function i.e., absolutely necessary for the stability of the chromosome.  Centromeres are required for the segregation of the centromere during meiosis and mitosis.  Teleomeres provide terminal stability to the chromosome and ensure its survival

Centromere  The region where two sister chromatids of a chromosome

appear to be ed or “held together” during mitatic metaphase is called Centromere.  When chromosomes are stained they typically show a dark-stained region that is the centromere.  Also termed as Primary constriction  During mitosis, the centromere that is shared by the sister chromatids must divide so that the chromatids can migrate to opposite poles of the cell.  On the other hand, during the first meiotic division the centromere of sister chromatids must remain intact  whereas during meiosis II they must act as they do during mitosis.  Therefore the centromere is an important component of chromosome structure and segregation.

Centromere  As a result, centromeres are the first parts of

chromosomes to be seen moving towards the opposite poles during anaphase.  The remaining regions of chromosomes lag behind and appear as if they were being pulled by the centromere.  The centromere consists of short repeated DNA sequences that are A-T-rich, known as a satellite DNA.

Chromosome Types: Based on Centromere Position Chromosomes are divided into four types based on the centromere position.

 Metacentric Chromosome  Centromere is located exactly at the centre of chromosome, i.e. both arms are equal in size. Such chromosomes assume ‘V’ shape at anaphase.

 Submetacentric Chromosome  The centromere is located on

one side of the centre point such that one arm is longer than the other.  These chromosomes become ‘J’ or ‘L’ shaped at anaphase

 Acrocentric Chromosome  Centromere is located close to one

end of the chromosome and thus giving a very short arm and a very long arm.  These chromosomes acquire ‘ J’ shape or rod shape during anaphase.

 Telocentric Chromosome  Centromere is located at one end of the chromosome so that the chromosome has only one arm.  These chromosomes are ‘I” shaped or rod shaped.

 The centromere divides the chromosome into two

arms, so that, for example, an acrocentric chromosome has one short (p) and one long arm (q arm).  The p arm is named for "petit" meaning 'small'; the q arm is named q simply because it follows p in the alphabet.  While, a metacentric chromosome has arms of equal length.  All house mouse chromosomes are acrocentric, while human chromosomes include both metacentric and acrocentric, but no telocentric.

Nos of centromere  Monocentric, Dicentric, Acentric(without

centromere), Polycentric- nos of centromere.

 Holocentric Chromosome

In holocentric chromosomes, the centromere runs through the entire length of the chromosome.  These chromosomes are very common in cells belonging to organisms in the animal and plant kingdom.  Include nematodes, such as Ascaris, insects, such as Lepidoptera, and plants in the genus Lazula.

Kinetochore  Within the centromere region, most species have

several locations where spindle fibers attach, and these sites consist of DNA as well as protein.  The actual location where the attachment occurs is

called the kinetochore and is composed of both DNA and protein.  The DNA sequence within these regions is called CEN

DNA.

A model of centromere structure

 The kinetochore contains two regions:  an inner kinetochore plate, which is tightly

associated with the centromere DNA.  an outer kinetochore plate , which interacts with spindle microtubules.  The kinetochores do not form part of the chromatid

but lie one on each side of the chromosome such that each chromatid is having its own kinetochore.  One kinetochore is attached to the spindle fibres towards one pole and the other similarly towards the other pole.

Telomere

 The two ends of a chromosome are known as telomeres.  It required for the replication and stability of the chromosome.  Establish chromosome positioning  When telomeres are damaged or removed due to chromosome breakage, the damaged chromosome ends can readily fuse or unite with broken ends of other chromosome.  Thus it is generally accepted that structural integrity and individuality of chromosomes is maintained due to telomeres.

Telomere Repeat Sequences Until recently, little was known about molecular structure of telomeres. However, during the last few years, telomeres have been isolated and characterized from several sp.

Species Arabidopsis Human Oxytricha Slime Mold Tetrahymena Trypanosome

Repeat Sequence TTTAGGG TTAGGG TTTTGGGG TAGGG TTGGGG TAGGG

Sequence in telomere is highly conserved TTAGGG (Homo sapiens)

Likely functions of telomeres: •Maintain structural integrity-loss of a telomere can result in fusion with another broken chromosome or can be degraded. •Establish chromosome positioning •Ensure complete replication. The end replication problem is solved by telomerase, an RNA-protein enzyme. Telomerase is a reverse transcriptase - RNA-dependent DNA polymerase - carries internal RNA component needed to prime the leading strand and provide the template for the lagging strand.

Secondary constriction  Besides primary constriction , secondary constriction can

also be observed in some chromosomes, which if present in the distal region of the arm, would pinch off a small fragment called Satellite.

 The satellite remains attached to the rest of the body of

chromosomes by a thread of chromatin.

 The secondary constrictions are always constant in their

positions and hence can be used as markers.

 The chromosomes having a satellite are marker/ satellite

chromosomes and are also called SAT chromosomes.

Satellite Chromosome  It is the chromosome which has a bulge on the telomeric end and contains the enzyme Sine Acido Nucleinico.  Also referred to as SAT Chromosome, it plays a vital role in the formation of the nucleolous after division is completed.  Chromosome may possess secondary constriction in one or both arms of it.  It shows repetitive sequences of genes

 Secondary constrictions are useful in identifying a chromosome from

a set. There are either 0, 1, 2, 3, or 4 secondary constriction sites in a cell at anaphase.  Some parts of these constrictions indicates sites of nucleolus

formation and so they are called "Nucleolar Organizing Region"  The formations of nucleolus takes place around the NOR region.  The secondary constriction also contains the genes for rRNA

synthesis (18s, 5.8s, 28s rRNA)  NOR occurs in SAT (satellite chromosome) chromosomes (13,14,15,21,22)

Chromsome Number  Human cells are diploid and have 22 different

types of autosome, each present as two copies, and two sex chromosomes/allosomes. This gives 46 chromosomes in total.  The nos. of chromosome varies from spp. to spp. but nos. is fixed to a particular sp.

Chromsome Number

 Ploidy is the number of complete sets of

chromosomes in a biological cell.

 In humans, the somatic cells are diploid but sex

cells (sperm and egg) are haploid. In contrast, tetraploidy is a type of polyploidy and is common in plants.

 Euploidy, or the euploid number, is a species

normal number of chromosomes per cell. For example, the euploid number of chromosomes in a human cell is 46.

Chromsome Number  Smallest number: The female of

an ant subspecies, Myrmecia pilosula, has one pair of chromosomes per cell. Its male has only one chromosome in each cell.

 Largest number: In the fern

family of plants, the species Ophioglossum reticulatum (fern) has about 630 pairs of chromosomes, or 1260 chromosomes per cell.

 Ascaris megalocephala (horse

roundworm) 2n =2

Organism  Human  Chimpanzee  Dog  Horse  Chicken  Goldfish  Fruit fly  Mosquito  Nematode  Horsetail  Sequoia  Round worm

No. chromosomes 46 48 78 64 78 94 8 6 11(m), 12(f) 216 22 2

Organism  Onion  Mold  Carrot  Tomato  Tobacco  Rice  Maize  Haploppus gracilis  Crepis capillaris

No. chromosomes 16 16 20 24 48 24 20 4

6

Chromosome Size  In contrast to other cell organelles, the size of chromosomes shows a remarkable variation depending upon the stages of cell division.  Interphase: chromosome are longest & thinnest  Prophase: there is a progressive decrease in their

length accompanied with an increase in thickness  Metaphase: Chromosomes are the most easily observed and studied during metaphase when they are very thick, quite short and well spread in the cell.  Anaphase: chromosomes are smallest.  Therefore, chromosomes measurements are generally taken during mitotic metaphase.

 The size of the chromosomes in

mitotic phase of animal and plants sp generally varies between 0.5 µ and 32 µ in length, and between 0.2 µ and 3.0 µ in diameter.  The longest metaphase

chromosomes found in Trillium 32 µ.  The giant chromosomes found in

diptera and they may be as long as 300 µ and up to 10 µ in diameter.

 In general, plants have longer chromosomes than

animal and species having lower chromosome numbers have long chromosomes than those having higher chromosome numbers.  Among plants, dicots in general, have a higher number of

chromosome than monocots.  Chromosomes are longer in monocot than dicots.

Karyotype  The term karyotype is given to the group of

characteristics that identifies a particular chromosome set and is usually represented by a diagram called idiogram, where chromosomes of haploid set of an organism are ordered in a series of decreasing size.  The karyotypes of different groups are sometimes compared and similarities in karyotypes are presumed to represent evolutionary relationships.

Human chromosomes are divided into 7 groups & sex chromosomes A 1-3 Large metacentric B 4,5 Large submetacentric C 6-12, X Medium sized, metacentric and submetacentric D 13-15 medium-sized acrocentric plus satellites E 16-18 short metacentric 16 or submetacentric 17,18 F 19-20 Short metacentrics G 21,22,Y Short acrocentrics with satellites. Y no satellites.

 Karyotype also suggests primitive or advanced feature

of an organism.  A karyotype showing large differences between smallest and largest chromosome of the set and having fewer metacentric chromosomes, is called asymmetric karyotype, which is considered to be a relatively advanced feature when compared with symmetric karyotypes.

Fig. 6.10. (A) A symmetric and (B) an asymmetric karyotype

Chromosome banding  To see chromosomes by microscope, they are normally

treated with chemical dyes, such as Giemsa.  The chromosome will appear as a series of alternate dark and light bands.  If Giemsa is used, the dark band is called G-band or G-positive band, and the light band is named Gnegative band. .

 Similar banding patterns can be observed by using

another dye, Quinacrine.  However, if chromosomes were treated in a hot alkaline solution before staining with Giemsa, a reverse pattern will be observed, namely, the original dark band will become light band, and vice versa.  For this reason, the G-negative band is also known as the R-band

G-banded karyotype for Male - 550 band level